This invention relates to establishing a seal between two substrates, particularly where at least one of the substrates is glass.
Many applications exist where it is necessary to secure a glass substrate within a frame such as a metal, plastic or wood frame, which may be painted. For example, glass windshields are secured within the metal or plastic frame of a motor vehicle both during vehicle manufacture and following manufacture to replace the windshield in the event that it cracks or breaks.
It is difficult to establish a strong bond to glass using conventional sealants and adhesives such as polyurethane pastes. To enhance adhesion, the glass surface is typically primed prior to inserting it into the frame.
Polyurethane pastes are conventionally used to establish a seal between the primed glass and the frame. Such pastes, however, are difficult to apply uniformly and reproducibly. Another problem is that pushing the glass into the frame causes the paste to flow and squeeze out of the bond line. This creates bond lines of uneven thickness and glass-frame contact points that can act as failure points because any stress applied to the frame is transmitted directly to the glass at these points. This is particularly a problem when a motor vehicle windshield is installed into a frame that has a highly uneven surface. To address this problem, discontinuous xe2x80x9cspacersxe2x80x9d are typically placed at various points around the perimeter of the frame. While these spacers help avoid creating glass-frame contact points, they also act as stress concentration points because while the sealant shrinks during cure, the spacers do not. It is then necessary to use extra sealant to accommodate the spacers.
Another problem is encountered in the case of polyurethane sealant pastes that require a relatively long time to cure and build bond strength such as those which are moisture-curable. During this vulnerable curing period, the glass can vibrate within the frame, making the seal and the glass susceptible to damage. Gaps in the seal can form, giving rise to wind noise and compromising seal integrity. The noise associated with the vibrations is also undesirable. Moreover, the reliance on ambient moisture means that the cure process varies depending upon ambient conditions.
In a first aspect, the invention features an article (e.g., in the form of a tape) that includes (a) a conformable, compressible, melt flow-resistant foam core layer having first and second major surfaces, and (b) a thermosettable sealant layer on said first major surface of the core layer. The sealant layer has a surface available for contacting a substrate.
A xe2x80x9csealant compositionxe2x80x9d or a xe2x80x9csealant layerxe2x80x9d is a gap-filling material. Consequently, at the time of seal formation, sealant compositions according to the invention have an elasticity that is sufficiently low such that the sealant composition is able to flow into and fill gaps in the substrate to which it is applied and, after the sealant has cured (in the case of thermosetting sealant compositions) or solidified upon cooling (in the case of thermoplastic sealant compositions), still sufficiently fill the gaps so as to seal the substrate. Both the surface of the sealant layer available for contacting a substrate and the bulk composition of the sealant layer meet these criteria. Sealant compositions useful in the invention are non-tacky (i.e., they are not tacky to the touch) once they have cured (in the case of thermosetting sealant compositions) or solidified upon cooling (in the case of thermoplastic sealant compositions).
In addition, the sealant compositions do not meet the definition of a pressure sensitive adhesive as established by the Pressure Sensitive Tape Council (PSTC), Glenview, Ill. According to the PSTC Glossary of Terms (August, 1985 revision), pressure sensitive adhesives are aggressively and permanently tacky at room temperature and firmly adhere to a wide variety of dissimilar surfaces upon mere contact and without the need for more than finger or hand pressure. They require no activation by water, solvent or heat in order to exert a strong adhesive holding force toward materials such as paper, plastic, glass, wood, cement and metals. They have a sufficiently cohesive holding and elastic nature so that, despite their aggressive tackiness, they can be handled with the fingers and removed from smooth surfaces without leaving a residue.
A xe2x80x9c""thermosettingxe2x80x9d or xe2x80x9cthermosettablexe2x80x9d composition is one which can be cured (i.e., crosslinked), for example by exposure to, preferably, heat or actinic radiation (although exposure to moisture or other chemical means may also suffice), to yield a substantially infusible (i.e., thermoset) material. Combinations of these various curing means may also be used (e.g., a combination of heat and actinic radiation). Such compositions may include a curing agent (e.g., a thermal or photo-active curing agent).
A xe2x80x9cthermoplasticxe2x80x9d composition is one which is capable of being repeatedly softened by heat and hardened by cooling.
A xe2x80x9cmelt flow-resistantxe2x80x9d material is a material that resists undergoing macroscopic mass flow under conditions at which the sealant layer exhibits macroscopic flow. Typically, the melt flow-resistant material resists undergoing macroscopic mass flow when subject to temperatures of up to about 200xc2x0 C.
A xe2x80x9cconformable, compressiblexe2x80x9d material is a material that readily deforms when subjected to an applied stress, but will tend to elastically recover when the stress is removed within the time frame that it takes to establish a seal between two substrates of interest, although some permanent set or deformation may occur depending on the stress to which the material is subjected in a given application.
In one embodiment, the thermosettable sealant layer includes a blend of (a) an epoxy resin, (b) a resin selected from the group consisting of polyacrylates, semi-crystalline polyesters, and combinations thereof, and (c) a curing agent selected from the group consisting of (i) thermally activated curing agents characterized by a thermal activation temperature and (ii) photo-active curing agents characterized by a thermal decomposition temperature.
In another embodiment, the thermosettable sealant layer substantially retains its shape when heated to a temperature greater than the softening temperature of the composition, but less than about 200xc2x0 C., until acted upon by an external force other than gravity. Such force includes the pressure exerted during sealing by pushing two substrates together. One test for determining whether a given composition exhibits this behavior involves placing a sample of the composition on a plate maintained at an angle in an oven, heating the sample to the desired temperature, and observing the extent to which the sample loses its initial shape and flows down the surface of the plate within a set period of time. Because the test is conducted in the absence of an applied external force, any such flow is attributable to the combined effect of temperature and gravity alone. This test is described in greater detail in the xe2x80x9cExamplesxe2x80x9d section below.
In another embodiment, the sealant layer includes a thermosetting sealant composition that includes a curing agent selected from the group consisting of (a) thermally activated curing agents characterized by a thermal activation temperature, and (b) photo-active curing agents characterized by a thermal decomposition temperature. The sealant composition is characterized in that, prior to cure, the composition substantially retains its shape when heated to a temperature greater than the softening temperature of the composition, but less than (a) the thermal activation temperature of the curing agent, where the curing agent is a thermally activated curing agent, or (b) the thermal decomposition temperature of the curing agent, where the curing agent is a photo-active curing agent, until acted upon by an external force other than gravity, measured according to the test procedure generally described above.
An example of a preferred sealant composition includes a blend of an epoxy resin, a semi-crystalline polyester, and a curing agent selected from the group consisting of (a) thermally activated curing agents characterized by a thermal activation temperature, and (b) photo-active curing agents characterized by a thermal decomposition temperature. The sealant composition is characterized in that, prior to cure, the sealant composition substantially retains its shape when heated to a temperature greater than the melting temperature of the polyester but less than (a) the thermal activation temperature of the curing agent, where the curing agent is a thermally activated curing agent or (b) the thermal decomposition temperature of the curing agent, where the curing agent is a photo-active curing agent, until acted upon by an external force other than gravity, measured according to the test procedure generally described above. Preferably, this sealant composition substantially retains its shape when heated to a temperature greater than the melting temperature of the polyester, but less than about 200xc2x0 C., until acted upon by an external force other than gravity.
The core layer preferably has a tensile strength no greater than the tensile strength of the sealant layer. Examples of suitable core layers include foams, which may be open or closed cell foams, although closed cell foams are preferred. Examples of suitable foams include polyacrylic, polyurethane and polyolefin foams. Also useful are core layers in the form of pressure sensitive adhesives, e.g., pressure sensitive adhesive foams.
The article may further include a bonding layer provided on the second major surface of the core layer. In such embodiments, the sealant layer and the bonding layer preferably are thermally insulated from each other.
In a second aspect, the invention features an article that includes (a) a conformable, compressible, melt flow-resistant thermoset core layer having first and second major surfaces, and (b) a thermosettable sealant layer on said first major surface of the core layer. The sealant layer has a surface available for contacting a substrate. The thermoset core layer may be provided by, for example, a closed cell foam or a pressure sensitive adhesive. Useful thermosettable sealant layers include those which were described above.
In a third aspect, the invention features an article that includes (a) a conformable, compressible, melt flow-resistant core layer having first and second major surfaces, and (b) a thermoplastic sealant layer on said first major surface of the core layer. The sealant layer has a surface available for contacting a substrate and is formed from a thermoplastic polymer selected from the group consisting of polyurethanes, polyesters, polyaromatic-containing block copolymers, and silicones. Useful core layers include those previously described.
In a fourth aspect, the invention features an article comprising (a) a conformable, compressible, melt flow-resistant thermoset core layer having first and second major surfaces, (b) a sealant layer on said first major surface of said core layer and having a surface available for contacting a substrate, and (c) a thermosettable bonding layer on said second major surface of said core layer and having a surface available for contacting a second substrate. Useful core and sealant layers include those materials previously mentioned.
In a fifth aspect, the invention features an article that includes (a) a substrate having a first major surface and a second major surface separated by an edge region having a finite thickness; (b) a conformable, compressible, melt flow-resistant core layer having first and second major surfaces; and (c) a thermosettable sealant layer provided on the first major surface of the core layer and having a surface available for contacting a second substrate. Examples of suitable core and sealant layers include the materials described above. The core layer is affixed at its second major surface to (i) the first major surface of the substrate and/or (ii) the edge region of the substrate. The core layer imparts vibration damping properties to the article.
The invention also features a method for joining these articles to a second substrate by contacting the sealant layer with the second substrate to join the second substrate to the first substrate through the sealant layer.
An example of a preferred substrate is glass, e.g., a glass windshield adapted for use in a motor vehicle. Such substrates can be joined to, e.g., metal substrates, painted substrates (e.g., painted metal substrates), and, in the case of windshields, frames of the type found in motor vehicles. Higher surface energy substrates are particularly usefully joined to each other. Another example of a second substrate is a U-shaped bracket into which the sealant-bearing article can be placed.
In one embodiment, particularly useful in the case of windshields or substrates installed in grooves, the first major surface of the substrate is characterized by a first perimeter, the second major surface of said substrate is characterized by a second perimeter, and the core layer is affixed at its second major surface to (i) the first major surface of the substrate such that the core layer extends substantially around the entire perimeter of the first major surface of the first substrate, and/or (ii) the edge region of the substrate such that the core layer substantially surrounds the edge region.
In a sixth aspect, the invention features an article that includes (a) a substrate having a first major surface characterized by a first perimeter and a second major surface characterized by a second perimeter, in which the first and second surfaces are separated by an edge region having a finite thickness; (b) a conformable, compressible, melt flow-resistant core layer having first and second major surfaces; (c) a sealant layer provided on the first major surface of the core layer; and (d) a second substrate joined to the first substrate through the sealant layer. The core layer is affixed at its second major surface to (i) the first major surface of the first substrate such that the core layer extends substantially around the entire perimeter of the first major surface of the first substrate, and/or (ii) the edge region of the substrate such that the core layer substantially surrounds the edge region. The core layer imparts vibration damping properties to the article. Examples of suitable core and sealant layers include the materials described above.
In one preferred embodiment, the first substrate includes glass and the second substrate includes metal. In a second preferred embodiment, the first substrate includes glass and the second substrate includes a painted substrate (e.g., a painted metal substrate). In one particularly preferred embodiment, the first substrate is a glass windshield and the second substrate is a frame (e.g., formed in a motor vehicle) for supporting the windshield.
In a seventh aspect, the invention features a sealant composition that includes a blend of an epoxy resin, a semi-crystalline polyester, and a curing agent selected from the group consisting of (a) thermally activated curing agents characterized by a thermal activation temperature, and (b) photo-active curing agents characterized by a thermal decomposition temperature. The sealant composition is characterized in that, prior to cure, the composition substantially retains its shape when heated to a temperature greater than the melting temperature of the polyester but less than (a) the thermal activation temperature of the curing agent, where the curing agent is a thermally activated curing agent, or (b) the thermal decomposition temperature of the curing agent, where the curing agent is a photo-active curing agent, until acted upon by an external force other than gravity, measured according to the test procedure described generally above.
In preferred embodiments, the sealant composition further includes a thixotropic agent, e.g., selected from the group consisting of particles (such as silica particles), chopped fibers, bubbles (such as glass, ceramic or polymeric bubbles), and combinations thereof Prior to cure, the composition preferably substantially retains its shape when heated to a temperature greater than the melting temperature of the polyester, but less than about 200xc2x0 C., until acted upon by an external force other than gravity.
The invention provides an easy-to-use sealant in the form of an article such as a tape for establishing a seal between two substrates that is particularly useful where at least one of the substrates is glass. The sealant and one of the substrates may be provided in the form of a single, ready-to-use article. The sealant can be applied uniformly and consistently, and does not excessively squeeze out when the substrate is pushed into a frame. Thus, clean-up following the sealing operation is simplified. The sealants may also be used without a primer.
Once placed between two substrates the preferred sealants build strength quickly, resulting in a seal having good green strength. Thus, it minimizes or eliminates the need for special precautions to support one, or both, of the substrates during the sealing operation. The rapid build-up of strength also eliminates problems relating to stresses imposed on the substrate prior to full cure such as may be caused by movement of the substrate relative to the frame. Thus, for example, in the case of windshield installation, it is possible to drive away in the vehicle bearing the newly installed windshield before cure is complete.
The ability to build green strength rapidly, coupled with the ability to eliminate processing steps such as priming and cleaning up excess sealant squeezed out of the bond line, simplifies the sealing process. This, in turn, facilitates use of the sealants in a motor vehicle assembly line. In addition, this imparts greater flexibility to the motor vehicle assembly process. For example, instead of installing the windshield early in the manufacturing process to allow time for sealant cure before the vehicle is driven off the manufacturing line, it becomes possible to install the windshield late in the manufacturing process.
The preferred sealants can be stored for extended periods of time without degrading because cure does not commence until the composition is exposed to heat or actinic radiation. Advantageously, the preferred heat- or actinic radiation-curable sealants cure relatively independently of ambient conditions that could limit the utility of temperature- and humidity-sensitive materials such as moisture-curable sealants.
Following cure, the sealant forms a tough, ductile material having good tensile strength. Thus, it maintains a good seal between the substrate and the frame even when the seal is subjected to ambient moisture and stress, e.g., of the type encountered during motor vehicle use. In addition, the sealant exhibits low shrinkage upon cure, thereby maintaining the seal and minimizing stress to the substrate. Particularly in the case of glass substrates, such stresses can cause the glass to crack.
The compressible, conformable core layer acts as an integral bond line spacer and forms a vibration damping cushion on which the substrate floats within the frame.
Because it preferably is substantially continuous around the perimeter of the substrate surface, it can advantageously accommodate and dissipate stresses to which the article is subjected under normal use conditions. An additional advantage is that the preferred constructions, under high shear rate catastrophic impact, may transmit the imposed stress to the substrates. In addition, the compressible, conformable property of the core layer allows for greater sealing capacity, thus reducing the amount of sealant needed and minimizing squeeze out.
Other features and advantages will be apparent from the following description of the preferred embodiments thereof, and from the claims.